Mobile phones equipped with a touch panel have been widely used, and a glass tempered by an ion exchange or the like (so-called tempered glass) tends to be used for the cover glass of mobile phones. Because a tempered glass is high in mechanical strength compared with an untempered glass, the tempered glass is suitable for the above-mentioned application (see Patent Document 1 and Non-patent Document 1 below).
In recent years, touch panels are being equipped for applications other than mobile phones as well, and hence, exterior parts each having a specific shape such as a curved shape are necessary depending on their applications. In order to adopt a tempered glass to those applications, it is necessary to subject the tempered glass to processing so that the tempered glass has a specific shape such as a curved shape. A tempered glass having a specific shape can be produced by forming molten glass into obtain a glass having, for example, a plate shape, then modifying the shape of the glass to a specific shape by thermal processing, and, after that, performing a tempering treatment (see Patent Documents 2 and 3 below).
Thus, those tempered glasses are required to have characteristics such as excellent thermal processability, that is, a low softening point, as well as a high mechanical strength.    Patent Document 1: JP 2006-83045 A    Patent Document 2: U.S. Pat. No. 7,168,047    Patent Document 3: JP 2001-247342 A    Non-patent Document 1: Tetsuro Izumitani et al., “New glass and physical properties thereof”, First edition, Management System Laboratory. Co., Ltd., Aug. 20, 1984, pp. 451-498